Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient's body.
Typically, an x-ray tube includes an evacuated envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination. In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly. The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the evacuated envelope. A cathode focusing cup housing the cathode filament focuses the electrons onto a small area or focal spot on a target of anode assembly. The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images to be constructed.
In order to distribute the thermal loading created during the production of x-rays a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam.
Typically, the anode assembly is mounted to a rotor which is rotated by an induction motor. The anode assembly and rotor are part of a rotating assembly which is supported by a bearing assembly. The bearing assembly provides for a smooth rotation of the anode assembly about its axis with minimal frictional resistance. Bearings disposed in the bearing assembly often consist of a ring of metal balls which surround and rotatably support the rotor to which the anode assembly is mounted. Each of the balls are typically lubricated by application of lead or silver to its outer surface thereby providing support to the rotating assembly with minimal frictional resistance.
Heat created by the anode assembly during the production of x-rays may be thermally radiated and transferred to the bearings rather than being absorbed by the oil or other cooling fluid in the x-ray tube housing. For instance, heat radiating from the anode assembly may become absorbed at an intermediate point along a path P1 (FIG. 1) leading between the anode assembly and the bearings and thus be transferred to the bearings. Unfortunately, such heat transfer to the bearings has been found to deleteriously effect the bearing performance. For instance, prolonged or excessive heating to the lubricant applied to each ball of a bearing can reduce the effectiveness of such lubricant. Further, prolonged and/or excessive heating may also deleteriously effect the life of the bearings and thus the life of the x-ray tube.
In order to reduce the amount of heat passed from the anode assembly to the bearings during operation, a heat shield is often mechanically secured to the rotor. The heat shield is typically threaded to the rotor or secured using screws or pinning. Although such a heat shield does provide protection to the bearings from the heating effects of the anode assembly, the mechanical mounting of the heat shield to the rotor presents some difficulties. For instance, cutting threads in the heat shield for securing to the rotor is often a tedious and difficult process. Further, loose particles or chips created by mechanically joining the heat shield to the rotor and/or shaken off during operation of the x-ray tube can deleteriously effect x-ray tube life and/or performance.
Therefore, what is needed is an apparatus for reducing the heating effects on x-ray tube bearings caused by heat dissipated from the anode assembly which overcomes the shortfalls discussed above and others.